Printing on a variety of surfaces is well known in the art. Printing has been done on paper, fabric, wood, and other surfaces for generations. Printing on newer synthetic materials, such as polymer films, is also known. Printing allows colors, graphic designs, and text to be placed on the material of interest.
Printing on polymer film and other extensible materials can present challenges, though, due to the extensible nature of the material. Extensible materials such as polymer films can stretch and deform when stressed, even if the material is not considered to be elastomeric.
For example, continuous webs such as films undergo stresses when being wound on a roll. These stresses on the web may vary depending on the depth of the web on the wound roll. For instance, material close to the roll core may experience a great deal of tension due to the force required to initially start winding the roll. The material closer to the middle of the roll depth may experience less tension as the roll is smoothly wound. The material near the outer portion of the roll may experience increased tension. Thus, as an extensible material is wound on a roll, these varying tensions can cause the material to stretch slightly to a lesser or greater extent. Some winders have the ability to adjust the winding speed and tension over the profile of the roll in order to compensate somewhat for this effect.
Another effect that extensible materials, particularly polymer films, can experience is relaxation during aging. This relaxation process is sometimes referred to as “snapback”. When a film is first extruded, the polymer chains may be aligned and stressed somewhat during the extrusion process. As the film cools, and particularly as it ages for a few days, these stresses are gradually released and the film relaxes. During this relaxation, the film will tend to retract (i.e., shorten) slightly. However, because of the varying web tensions within the roll itself, varying degrees of retraction will be observed within the roll.
These two problems, winder tension variability and snapback, can cause a printed extensible materials to vary significantly in print repeat length. A pattern printed repeatedly on a strip of extensible material can be distorted by as much as 1% in length as the printed material is wound and later as it ages. This distortion, particularly if cumulative, can result in misaligned or miscut product when the printed material is later unwound for converting.
Today's consumer has come to expect high-quality, detailed graphics on products from packaging films to shrink-wrap seals to disposable hygiene products. There is thus a continuing need to improve the printing repeat-length control of polymer films in order to manufacture materials that can meet this expectation.